1. Field of the Invention
The present invention generally relates to a gallium nitride (GaN) type semiconductor device comprising a (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N single crystal, which has been receiving growing attention as a wide gap semiconductor in the industry, and a method of fabricating the same. More particularly, this invention is directed to a method of forming on a silicon (Si) substrate a high-quality (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N single crystal, which is expected to be a material for a device for emitting or detecting light with a wavelength of 200 to 700 nm.
2. Description of the Related Art
A (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N crystal (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, excluding the case of x=1 and y=0) is a direct transient semiconductor with a light wavelength of 200 to 700 nm to the energy band gap at the room temperature, and is expected as a material particularly for a device which emits and receives light having a wavelength of a visible short wavelength range or an ultraviolet range.
Since the (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N crystal contains nitrogen (N) whose equilibrium vapor pressure is very high near the growth temperature, its bulk crystal cannot be easily produced. At present, therefore, the single crystal is produced through a hetero epitaxial growth where different crystals are used as a substrate.
As the essential requirements for a substrate for producing the (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N crystal, it is desirable that the substrate have the following characteristics:
(1) High melting point (at least 1,000.degree. C.)
(2) Chemically stable
(3) Excellent crystal quality
(4) Small difference in lattice constant
(5) Easy to obtain
(6) Large in size
In addition, in the case of fabricating a device which electrically functions:
(7) it is desirable that the electric characteristics be easily controlled; particularly, a low resistance is desirable.
No crystals which satisfy all of those conditions have been found, yet. The substrate which is most frequently used at present is formed of sapphire that satisfies the conditions (1), (2), (3), (5) and (6).
Since the difference between the lattice constant of sapphire and that of (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N is 11% or more, however, sapphire is not desirable in view of the condition (4). In addition, because sapphire is an insulating material and is rigid, it is not easy to form a gallium nitride type semiconductor device, particularly, its electrode, it is not suitable for a semiconductor device that operates on a large current supplied.
Silicon (Si) is one of proposed substrate materials to overcome this shortcoming. By using Si, a preferable substrate is obtained with a low resistance and high melting point and having a large and substantially perfect crystal. A Si substrate is inexpensive and has a high crystal quality, and large crystallinity. The fine working of the Si substrate are easily accomplished. That is, Si fulfills the conditions (1), (2), (3), (5), (6) and (7). The biggest issue in yielding a (Ga.sub.1-x Al.sub.x).sub.1-y In.sub.y N crystal on a Si substrate is that there is a large difference in lattice constant of about 17% between GaN and Si, and thus there is a demand for establishment of the technology to suppress the occurrence of crystal defects due to this difference in lattice constant.